I brew frequently with molasses, treacle and Lyles Golden Syrup.
I believe you could put them in the boil at any time, but I prefer to add them the last 15 min. of the boil to prevent any scorching.

Molasses and treacle add a very prominate flavor to the final product. I usually add no more than .5 lb to any recipe. I've been hesitant to use any more than that because I don't want it to over power the malt flavor. But you may find that you like that result. I've seen several recipes that use up to 2 lb.

Molasses (or alternatively, brown sugar) is great in a porter and even in some special bitters...but heed Legman's advice; it can be intense so you don't want to go overboard. You may have to experiment to find the right threshold for the profile you're after. But it does really compliment a good porter. Sugars like this definitely have a place in quality brewing. The all-grain evangelists aside, some of the best beers in England, present and past, use various sugars such as this.